1. Field of the Invention
The present invention relates to a reducer of an electric power steering apparatus and, more specifically, to a reducer of an electric power steering apparatus which improves the formability and the accuracy of a worm wheel and increases the mechanical strength and the durability of the worm wheel when a high-energy electric power is transferred to a steering shaft from a worm shaft through the worm wheel, thereby being stably used for a long time.
2. Description of the Prior Art
In general, a power assisted-steering apparatus mounted on a vehicle includes an electric power steering apparatus in which a steering power is controlled by a motor. The electric power steering apparatus accurately drives a motor in an electronic control system according to a vehicle's driving condition detected by various sensors such as a speed sensor and a steering torque sensor, and a torque generated by the motor is transferred to a steering column or a pinion through a reducer so as to assist a steering power of a driver who operates a steering wheel connected to the steering column and the pinion.
Accordingly, the electric power steering apparatus provides a steering performance which enables a driver to maintain an optimum steering condition. For example, the electric power steering apparatus provides a light and comfortable steering state at the time of driving at a low speed and a heavy and stable steering state at the time of driving at a high speed and enables a driver in an accidental emergency situation to perform rapid steering to cope with the emergency situation.
Such an electric power steering apparatus may be divided into various types according to a mounting position. A reducer including a motor is usually installed at a steering column area for fixing a steering shaft, which connects a steering wheel of a driver's seat to a gearbox in a lower portion of a vehicle, to a framework or at a gearbox area in which a pinion coupled to a rack bar is mounted. The electric power steering apparatus allows a rotational force of a steering wheel which is operated by a driver to be transferred to a rack bar by the motor and the reducer.
FIG. 1 is a cross-section which illustrates an inner structure of a reducer of a conventional electric power steering apparatus.
A conventional reducer includes a worm shaft 104 made of a metal such as steel and having a worm 102 formed in the center portion of the worm shaft, worm bearings 106 are installed on both ends of the worm shaft 104, respectively, to support the worm shaft 104, and the worm shaft 104 is connected to a motor shaft 108 such that the worm shaft 104 is rotated by the driving of a motor 110.
A worm wheel 114 made of, for example, a resin composition, is placed on one side of an outer diameter of the worm 102 so as to be engaged with the worm 102, as a reduction gear, formed in the center portion of the worm shaft 104, and the worm wheel 114 is mounted on a steering shaft 112 for transferring a rotational force of a steering wheel (not shown) which is operated by a driver, such that a rotational force of the worm shaft 104, by the driving of the motor 110, is transferred to the steering shaft 112. The worm wheel 114 and the worm shaft 104 engaged by a worm-gear scheme are mounted in the inside of a housing 116 to be protected from the outside.
An electronic control device (not shown) mounted on a vehicle controls the driving of the motor 110 according to a driving condition of the vehicle, and a rotational force of the worm shaft 104 by the driving of the motor 110 is added to a rotational force of the steering wheel which is operated by a driver and is transferred to the steering shaft 112, thereby maintaining a driver's steering-driving condition to be smoothly and stably.
In this case, the worm wheel of the reducer as described above reduces the number of rotations of the motor and transfers the reduced rotation to the steering shaft. As described above, a gear made of a resin composition and a gear made of a metal may be formed in a pair.
However, since the conventional reducer has a structure in which a worm shaft, a worm wheel, and a steering shaft which rotate are engaged with each other, the vibration and noise generated by rotation may be directly and very loudly transferred to a driver and the durability of internal components of the reducer may decrease or the components may be damaged in extreme cases.
Especially, when driving on a bumpy road, a load transferred from a wheel through a rack bar and a steering shaft instantaneously increases, and as a result of the increment, the vibration and noise generated in the reducer may further increase to cause discomfort to a driver.